tampons can definitely be left in whilst showering...even if you want to go swimming they hold up but need changing soon after. the only time tampons don't hold up are ifyou are incontinent and wearing 'nappy' style pads for that as the urine goes in the vagina as it spreads through the pad before being absorbed and soaks into the bottom end of the tampon!
straight after wiping yourself, place your pointer finger gently in your vagina and gently push the tampon back up into place that way it should not be low enough to be felt or irritate. if it still is too low then maybe you need to look at using shorter ones, teen ones tend to me smaller than regular ones as they are designed for girls who are still growing.